1. Technical Field
This application relates generally to the field of lighting. More particularly, this application relates to the technology of controlling electrical loads, such as the intensity (i.e., dimming) of lighting sources.
2. Background Information
Presently, there are a variety of lighting sources in widespread commercial use. Some popular examples include incandescent, fluorescent, and solid state (e.g., light emitting diode (LED)) lighting sources. Even within certain lighting categories, there can be further distinctions, such as incandescent lighting operating at AC line-voltage levels (e.g., 120V, 60 Hz), or at DC low voltage (e.g., 6, 12, or 24 volts). Lighting sources operating at DC low voltages can be further distinguished into those using magnetic transformers and those using electronic (e.g., solid state) transformers. LED lighting sources typically require a matched LED driver, or power supply, providing the appropriate driving current and voltage levels dependent upon the nature of the LED lighting source.
In many lighting applications it is desirable to provide some measure of control to allow for variability of one or more attributes of the lighting source beyond simply “on” and “off” For example, a dimmer control can be provided to otherwise control the power delivered to the lighting source to achieve desired illumination intensity. Each type of lighting source (load types) has individual characteristics that generally require special types of dimmers. It is important to use a dimmer that is designed, tested, and UL listed for the specific lighting source/load type.
Dimmer controls can be user accessible, for example, as in wall switch styles providing a user adjustable control, such as a rotary knob, a sliding switch and electronically controllable switches (e.g., capacitively coupled). A user adjustment of the control is automatically converted by the dimmer into a corresponding power adjustment, for example, allowing a continuous adjustment of the resulting illumination from a maximum power (e.g., full on) to a minimum power (e.g., off). As a consequence of fundamental differences between the various lighting sources, a dimmer for one might not work with another. Thus, a dimmer control suitable for incandescent lighting may not be suitable for fluorescent or solid state lighting sources.
One such class of dimmer controls is referred to as electronic low voltage (ELV) dimmer controls. ELV dimmer controls are suitable for adjusting electrical power (e.g., average or RMS power) distributed to a low-voltage lighting source powered by electronic transformers, and therefore its light output, by effectively “chopping” the AC voltage waveform that is supplied to the lighting source as power. The particular chopping format used in ELV dimmers is commonly referred to as “trailing edge dimming” or “reverse-phase control.” At its full brightness setting, the ELV dimmer control allows most, if not all, of the AC power waveform to pass through it, to power the light. As the dimmer control is adjusted to a dimmer setting, the second half period of each of the positive and negative portions of each AC power cycle is chopped by a value proportional to the position of an internal potentiometer. A dimmer setting results in a lower average (e.g., RMS) power over the period, resulting in corresponding reduction of illumination output. An example of such a chopped AC waveform is illustrated in FIG. 1.
A waveform of a typical AC power cycle is represented by a sine wave having a period T, (e.g., T=16.7 msec for a 60 Hz power). The period includes a positive half cycle and a negative half cycle. An example of a trailing edge chopping includes a sudden and precipitous change in the AC voltage to substantially zero volts, as shown by the vertical portions of the waveform occurring at T/2−τ and T−τ. The AC voltage sine wave resumes upon each zero crossing until the next chopped portion, the pattern repeating for each AC cycle. Adjustment of the user adjustable control changes the position of the chopped portion (e.g., τ), such that the resulting waveform has greater or lesser average power.
Unfortunately, such dimmers are generally not well suited for LED lighting sources. Such solid-state lighting applications generally include a power supply converting facility AC power to power suitable for the solid state lighting. In particular, for LED lighting the direction of current as well as its amplitude are controlled by such a power supply to provide desired illumination. As such, digital lighting applications are typically isolated from the AC mains by the presence of such a driving power supply. Accordingly, there is no assurance that providing an ELV chopped AC signal to a driving power supply associated with solid state lighting will result in the intended illumination setting, or dimming. In fact, there is no assurance that the solid state lighting will even operate as intended when powered by such a chopped AC waveform.